Using a large UK study of around 17,000 kids, researchers wanted to know how the trajectory of their early-life BMI affected their psychosocial development.

It's that "trajectory" word that caught my eye on this one. Instead of looking at how higher BMI levels correlate with psychosocial outcomes in kids, the researchers wanted to categorize kids by the way their BMI was changing. And that's why I say there are only four types of kids.

Using something called latent class analysis, which searches the data for common patterns of BMI trajectories, four patterns emerged.

Most kids – around 84% - had a stable BMI trajectory, that's the bottom line in the graph. There was also a small number of kids – 1% or so, who started off with high BMI and then got skinnier over time. Finally, there were two groups of kids whose BMI increased over time – one to a moderate degree, and one to a higher degree.

Many factors associated with which group a kid was in, but I'll note a couple of interesting ones. It's probably not surprising that minorities, and kids with a lower socioeconomic background were more likely to be in the BMI increasing groups. But smoking in pregnancy just about doubled the risk of being in the rapid increase group. And kids who skipped breakfast had around a 75% higher chance of being in that rapid rising group.

Interestingly, playing sports, watching TV, and drinking sugary drinks had no relationship to any of these trajectories, which is a bit of a surprising twist.

As you might expect, kids in the BMI increasing group tended to have a harder time psychosocially. By parental report, they had more emotional symptoms and problems with their peers. And the kids reported higher levels of unhappiness, low self-esteem, and body dissatisfaction.

All of this squares with our intuition, but I was curious about the differential effects of changing body weight on these factors. Like – do kids with a rising BMI have a worse self-image than those whose BMI has always been high? Unfortunately, kids with a stable, high, BMI did not emerge as a latent class in the analysis so that comparison will remain, for now, a mystery.

So what have we learned? Well, don’t smoke while you're pregnant, and make sure your kids eat breakfast every day. I guess our moms were right all along.

F. Perry Wilson is an Assistant Professor of Medicine at Yale University, a commentator for medpagetoday.com, and a data-science and statistics nerd. Follow him on twitter www.twitter.com/methodsmanmd.